The normal method of measuring blood pressure by non-invasive means entails the use of an inflatable cuff for location around the upper arm and some means for detecting the flow of blood at a location distal to the cuff. The cuff is inflated to a pressure at which the embraced tissue is compressed and all blood vessels therein, including the arteries, are occluded. The pressure in the cuff is then gradually reduced and the arteries start to open transiently to allow blood flow as soon as the cuff pressure falls below the peak systolic level. Thereafter, the arteries remain patent for an increasing proportion of the cardiac cycle as the cuff pressure is further reduced, until the pressure falls below the diastolic level. Following this last event, the arteries remain patent throughout the cycle and the flow of blood is determined only by the normal pressure cycle.
The distally located detecting means, which is commonly a stethoscope or an electronic equivalent, readily enables the first onset of blood flow to be detected at systolic pressure. However, at diastolic pressure the change in flow is much more subtle and difficult to detect because the flow approaches its unobstructed level at this time in an asymptotic manner. In the result, measurements of diastolic pressure are commonly inaccurate and of poor reproducibility.
In accordance with other proposals for measuring blood pressure, a cuff or equivalent device can be employed in association, at the same sit, with some means for detecting variations in the blood volume in the body zone embraced by the cuff as that zone is variably occluded. One such proposal (Wood et al, Staff Meetings of the Mayo Clinic, July 5, 1950, pages 398-405) suggests that systolic and diastolic pressure measurements are given by the cuff pressure when the detector respectively shows a first output and then a maximum output as the cuff pressure is continuously decreased from a level above systolic to one below diastolic. However, the results given with this proposal are such as to suggest no marked improvement over conventional methods of measuring blood pressure by non-invasive means. Again, the measurement of diastolic pressure is of poor accuracy.
An object of the present invention is to improve the above situation and this improvement rests on the finding that the blood volume in a body zone changes in two distinct phases when subjected to a progressive variation of externally applied pressure to or from a level causing occlusion of the zone, and that these two phases are respectively associated with transition of the external pressure through the systolic and diastolic levels.